The present invention relates to a biomass mill for pulverizing a wooden (ligneous) biomass such as wooden chips, wooden pallets, etc. to be used as a fuel for boilers.
In the boilers currently in operation, a coal is primarily used as a solid fuel. For the purpose of reducing discharge of CO2, it is now studied and attempted to use a wooden biomass as a type of fuel, which has little burden on the environment.
When the wooden biomass is used as the fuel for boilers, the wooden biomass such as wooden chips, wooden pellets, etc. must be pulverized so that the wooden biomass will be combustible by burners.
When the wooden biomass is mixed in coal and is used as fuel, and if a mixing quantity of the wooden biomass is not much, it can be mixed and pulverized by using an existing type of coal mill. However, if the quantity of the wooden biomass used is increased, it is necessary to pulverize the wooden biomass by itself.
When a device for pulverizing the wooden biomass is manufactured, if it is designed as the device for pulverizing based on a coal roller mill for pulverizing coal, it would be possible to manufacture the device at low cost without extensively improving and without substantially changing manufacturing facility.
First, referring to FIG. 4, description will be given on a vertical mill 1 for pulverizing coal.
A closed space is formed by a casing 3, which is erected on a base 2, and a pulverizing table 5 is mounted in a lower portion of the space via a table drive unit 4. The pulverizing table 5 is rotated at constant speed by the table drive unit 4. On an upper surface of the pulverizing table 5, a table segment 7 is provided, which has a recessed groove 6 with a cross-section in arcuate form.
The necessary sets, e.g. 3 sets, of pressure roller units 8 are arranged in radial direction from a rotation center of the pulverizing table 5. The pressure roller unit 8 has a pressure roller 9, which can be tilted freely with a horizontal support shaft 11 as a center. In a lower portion of the casing 3, 3 sets of roller pressure devices 12 are disposed so that the roller pressure devices penetrate the casing 3 in radial direction. Each pressure roller 9 is pressed into the recessed groove 6 by the roller pressure device 12.
Under the pulverizing table 5, a primary air chamber 13 is provided. Above the pulverizing table 5 inside the casing 3, a classifier chamber 14 is arranged.
In the lower portion of the casing 3, a primary air supply port 15 is mounted, and the primary air supply port 15 is communicated with the primary air chamber 13. Around the pulverizing table 5, a gap is formed, and the gap fulfills a function as a primary air blowout hole 16.
In an upper part of the casing 3, a coal supply and discharge unit 17 is disposed. A chute 18 in form of pipe is provided so that the chute 18 penetrates a central portion of the coal supply and discharge unit 17, and the chute 18 is extended to inner space of the casing 3. The coal is supplied to the chute 18, and the supplied coal is to fall down to the pulverizing table 5.
A classifier 19 is rotatably mounted at the middle of the chute 18. The classifier 19 has a strip-type blade 21, which is disposed with a predetermined pitch in circumferential direction, and the classifier 19 is rotated by a rotary drive unit 22.
A pulverized coal feeding pipe 23 for feeding the pulverized coal to the boiler burner is connected to the coal supply and discharge unit 17.
Now, description will be given on pulverizing operation of coal at the vertical mill 1.
Under a condition that the pulverizing table 5 is rotated and the primary air is introduced via the primary air supply port 15, the coal in form of lump is sent through the chute 18. The coal in form of lump fall toward a center of the pulverizing table 5 from a lower end of the chute 18, and the coal is supplied onto the pulverizing table 5.
The coal on the pulverizing table 5 is moved in outer peripheral direction by centrifugal force. The coal is then caught by the pressure roller 9, is pulverized and is turned to powder state. Further, the coal is filled up in outer peripheral region by the centrifugal force from the table segment 7. The pulverized coal thus filled up is moved upward by the primary air, which is blown upward via the blowout hole 16.
On a peripheral region of the table segment 7, a dumb-ring 24 or an inner edge 26a is disposed in order to hold the coal on the table segment 7 for a required time period and to increase a pulverizing efficiency, and a coal layer with a thickness as required is formed on an upper surface of the table segment 7.
When the wooden biomass such as wooden chips, wooden pellets, etc. (hereinafter referred as “wooden biomass”) is supplied to the vertical mill 1 or to a mill with an equivalent structure and is pulverized, it is found that the biomass exhibits behavior different from behavior of the coal described as follows, and that the pulverizing efficiency cannot be attained sufficiently.
Because the coal has no caking property and has high specific gravity, when the coal is pulverized, the coal is moved smoothly by the centrifugal force caused by the rotation of the pulverizing table 5, and the pulverizing efficiency as desired can be attained. Similarly, sufficient pulverizing efficiency can be accomplished when a mixing ratio of the wooden biomass is 5% or less.
However, when the mixing ratio of the wooden biomass is increased or when the wooden biomass is pulverized by itself, the wooden biomass moves less smoothly on the table compared with a case of the coal because the wooden biomass is light in weight and is made of fibers. Therefore, a frequency of an engagement of the wooden biomass with the pressure roller 9 is low, and the pulverizing operation is not carried out effectively.
Further, when the wooden biomass is not moved smoothly toward outer periphery of the pulverizing table 5, the powder after pulverizing is not efficiently discharged out of the mill and the powder after pulverizing is stagnated in the mill. This causes an increase of differential pressure within the mill and an increase of the power of the table drive unit 4. Thereby, pulverizing capacity will be limited.